1. Field of the invention
The present invention relates, in general, to a heater for heating an automobile sensor and, more particularly, to an improvement in durability along with the heater and the ceramic heater.
2. Description of the Prior Art
Owing to superior endurance at high temperature, ceramic heaters are useful for a plurality of purposes, especially, for automobile sensors. An automobile has a solid electrolyte air/fuel (A/F) sensor for detecting the oxygen content contained in an exhaust gas from the engine, in order to purify the exhaust gas and to enhance the fuel consumption ratio. Such sensor can respond only when it is heated to at least about 800 degrees centigrade.
For the respondence, a heater is adopted to elevate the temperature. The heater is generally made by constructing an electrode on a substrate with high thermal conductivity. The electrode consists typically of platinum in combination with additives. Recently, there has been a strong demand for low-resistant heaters which are capable of elevating temperature more rapidly. The low resistance can be accomplished by incorporating a variety of ceramic powders in the heater.
When Al.sub.2 O.sub.3, a typical material for the substrate, is sintered, sinter-aiding agents and/or grain-growth inhibitors are usually added because of its high sintering temperature Oxides, such as SiO.sub.2, MgO, CaO and the like, are used as these additives because it is easy for them to make Al.sub.2 O.sub.3 glassy upon sintering. Other additives exist for these purposes. For example, oxides of Periodic Table Group IIIa metal, for example, Y.sub.2 O.sub.3, are added for reducing the sintering temperature and ZrO.sub.2 for inhibiting grain growth of Al.sub.2 O.sub.3. However, heaters employing such Al.sub.2 O.sub.3 substrates, when used at high temperature under direct current high voltage (e.g. automobile power 12V), allow the alkaline metal ions contained in the substrates, that is, Mg.sup.2+, Ca.sup.2+, to migrate into the negative terminal and thereby to give rise to segregation. As a result, compounds with low melting points are produced, which cause cracks on the surface of the heater and thus, short circuits in the heater electrode.
In order to prevent the migration, a variety of techniques have been undertaken, e.g. high purity of Al.sub.2 O.sub.3 or small amount of Al.sub.2 O.sub.3 in Pt. However, these techniques can relieve the migration only to some degree and the purer Al.sub.2 O.sub.3 requires a higher sintering temperature.
Referring to FIG. 1, there is shown a heater which is presently commercially available. As shown in this figure, it consists of two laminating substrates 1, 5 between which a migration pattern 2, a heater substrate 3 and a heater electrode 4 are, in sequence, formed. The migration pattern 2 is interposed between the laminating substrate 1 and the heater substrate 3, with the aim of preventing the generation of cracks on the heater substrate 3 and short circuits in the heater electrode 4. This heater, however, is disadvantageous in that its construction is done by complicated processes, which results in high production cost.